Ethylene is an extremely valuable commodity chemical for producing various chemical and polymer products used in numerous commercial as well as consumer products and applications. Ethylene may be produced in a number of petrochemical processes, including methanol-to-olefins (MTO) processes, fluid catalytic cracking processes (FCC), as well as thermal and steam cracking processes. These processes typically result in an effluent containing a mixture of hydrocarbons, as well as one or more of nitrogen, carbon dioxide, nitroxides, methane, ethane, and other hydrocarbons.
Before the ethylene produced can be sold and used, it is necessary to employ a process which recovers the ethylene component in a desirable, ethylene-rich stream by separating it from other components and impurities. Many times this separation is integrated with existing olefins plants but in certain instances, such as where off-gas flow rates are large enough, stand-alone units have also been operated. Because of the high quantity of lighter components such as hydrogen, nitrogen, and methane, the feed gases are typically compressed from pressure of about 1.17 to 1.38 MPa gauge (170 to 200 psig) to pressures around 3.45 MPa gauge (500 psig) in multi-stage feed gas compressors. The compression step allows for the recovery of 90% to 99% of the ethylene and heavier materials contained in the feed gases using a combination of mechanical refrigeration and expansion of the methane and lighter portions of the feed gas after demethanization. However, the capital and operating costs for the feed gas compressors are very high.
Further, the processing of refinery off-gases for olefin recovery has associated safety concerns because nitrogen oxide is also present in trace amounts in the refinery offgas streams. The nitrogen oxide easily oxidizes forming nitrogen dioxide. Nitrogen oxides, for example NO and NO2, are commonly referred to as NOx. Mixtures of nitrogen oxide and nitrogen dioxide can form dinitrogen trioxide (N2O3) at temperatures below −21° C. N2O3 and heavier diolefins (C4+) can react at these low temperatures forming nitrated gums which are unstable and can explode if thermally or mechanically shocked.
A typical process for low pressure olefins recovery from fluid catalytic cracker (FCC) offgas is disclosed in U.S. Pat. No. 5,502,971, which is hereby incorporated in its entirety. U.S. Pat. No. 5,502,971 describes a low pressure cryogenic technique for recovering C2 and heavier hydrocarbons from a refinery off-gas by eliminating feed gas compression and high pressures while maintaining recovery of C2 and heavier hydrocarbons at temperatures above temperatures at which nitrated gums can form.
One process for the separating and recovering of ethylene from a process effluent involves the use of flash stages and distillation at cryogenic temperatures, as described in U.S. Pat. Nos. 7,166,757 and 4,499,327. As described therein, the current state of the art ethylene recovery and separation processes which dominate the industry involve cryogenic boiling point separation of ethylene and methane at temperatures that may be lower than −90° C. The cryogenic separation can be very expensive due to both the capital cost of the specialized vessel metallurgy and refrigeration equipment, and the operating costs, including compression and cooling for the energy-intensive chill train.
As discussed, the use of cryogenic temperatures during the processes for treating refinery off-gas or process effluents can result in unstable and potentially dangerous operating conditions. For example, the NOx present in the refinery off-gas can react to form N2O3. Further, it has been found that the N2O3 formation rate significantly increases with decreasing temperature, thus making a cryogenic process especially susceptible. N2O3 is a highly oxidative compound, which can form highly unstable and highly reactive gums upon contact with poly-unsaturated compounds, such as butadiene. Even at cryogenic temperatures and at concentrations in the ppb levels, such unstable gums can accumulate and cause dangerous runaway reactions and even explosions.
Accordingly, there exists a need for an improved method of separating methane to recover ethylene and other valuable products from refinery offgas that reduces the capital and operating costs and improves the operation safety and stability.